In an optical transmission system, the signal transmission performance is affected by various transmission impairments of transmission links such as chromatic dispersion, non-linear effects, and polarization mode dispersion (PMD). In recent years, with the rapid development of the code type technologies for the optical transmission system, novel modulation formats capable of effectively reducing the influence caused by impairments of the transmission links have come into being. It is generally recognized that a DQPSK as a vector modulation format reported in literature during OFC2002 is the most promising modulation format for transmission systems at a transmission rate higher than 40 Gb/s. Researches show that, the DQPSK has a very small spectrum width and realizes a channel spacing of 12.5-25 GHz in DWDM systems of 10 Gbit/s; as compared with known modulation formats for optical modulation systems such as Non Return to Zero (NRZ) and binary differential phase shift keying (DPSK), at the same symbol rate, the DQPSK maintains the chromatic dispersion tolerance unchanged, but the system capacity thereof is twice of that of the modulation formats such as NRZ and DPSK; since the DQPSK can realize constant-envelope transmission or near constant-envelope transmission, various non-linear effects of the optical fibers, such as cross phase modulation (XPM) and self phase modulation (SPM), can be inhibited; and the DQPSK has a large symbol delay, and can improve the chromatic dispersion tolerance and PMD tolerance as well as spectral efficiency.
In the prior art, as shown in FIG. 1, a DQPSK signal transmitting end uses a pre-coding module to pre-code two input data signals u and v, so as to generate two driving electric signals I and Q. The driving electric signals I and Q respectively drive an upper branch and a lower branch of a dual-parallel MZ modulator, such that Mach-Zehnder modulators (MZMs) modulate a real part and an imaginary part of an input optical carrier to generate a real-part signal and an imaginary-part signal. After passing through a 90° phase shifter, the imaginary-part signal is combined with the real-part signal to generate an optical DQPSK signal. A DQPSK signal receiving end receives the DQPSK signal, uses a splitter to split the DQPSK signal into an upper optical signal and a lower optical signal, uses two Mach-Zehnder interferometers (MZIs) to respectively demodulate the upper optical signal and the lower optical signal of the DQPSK, and then uses two balanced receivers to recover signals u and v from the optical signals, thereby completing the receiving and demodulation of the signal.
Through making researches, the inventors found that the prior art at least has the following problems.
Since the DQPSK signal receiving end uses two MZIs and two balanced receivers to demodulate the DQPSK signal, the demodulation manner is rather complex, and has a high cost. In addition, since the MZIs have poor stability and are polarization dependent, the performance of the optical transmission system is greatly affected. Moreover, the higher the signal transmission rate is, the higher the accuracy requirements for the MZI will be, and the higher the cost for manufacturing the MZI will be.